Take five

Dwarf Brittle Star (Amphipholis squamata), found in the low intertidal zone on Bodega Head on 28 April 2013 and photographed under a microscope.  It was ~15 mm across.

 

 

 

 

 

I could have stopped with that series of five images, but I was fascinated when I caught a brief glimpse of the underside of the brittle star.  Here's a close-up (below).  The mouth is the dark cavity in the very center of the disk.  But what's perhaps most interesting in this particular image is the white scale with a single pore to the left of the mouth.

 

That modified scale is the madreporite (or sieve plate).  Water is pulled in through the madreporite and enters the water vascular system which powers the tubefeet.  In sea stars, the madreporite is on the aboral surface (opposite the mouth) — for an example, see the salmon-colored spot on the Ochre Sea Star in the photo below. 

 

But in brittle stars, the madreporite is on the underside, adjacent to the mouth.  It's not often as visible as it is here, so this is a nice illustration of one of the differences between a brittle star and a sea star.

Here's another view with the madreporite in a slightly different position.  Can you find it?

 

The madreporite is in the lower right corner (remember it's about halfway between the mouth and the outer edge of the disk).  [If the brittle star was a clock, the madreporite is located between the arm that points to 3 o'clock and the arm that points to 6 o'clock.]

Although this brittle star was small, it revealed a significant lesson!